The present invention is directed to time-measurment circuitry. It is directed particularly to digital measurement of very short time periods.
A convenient method of obtaining a digital duration measurement is to employ a high-frequency clock signal having a very stable frequency and to count the number of clock pulses that occur between two events whose separation in time is to be measured. A counter circuit counts the clock pulses, and the counter output is a digital quantity directly proportional to the duration of the event.
This method is repeatable and precise so long as the period of the clock is quite short with respect to the duration of the event. When the order of magnitude of the event duration approaches that of the clock period, however, the percentage error becomes significant, so one ordinarily increases the frequency of the clock as the duration to be measured becomes shorter. When the duration is extremely short, however, practical considerations preclude further increases in clock frequency.
Methods have therefore been proposed for modifying the pulse-counting method so as to increase its accuracy and resolution. These methods are directed to accurate measurement of the times between the first event and the first clock pulse thereafter and between the second event and first clock pulse after that. It is these times that the basic pulse-counting technique cannot measure, and it is these times that become significant when the duration to be measured is not large in comparison with the clock period.
One such method is described in U.S. Pat. No. 4,303,983 to Chaborski. The apparatus depicted in that patent employs a "time-to-amplitude converter," presumably a device for linearly charging a capacitor, to measure the time between the first event and the next clock pulse. A time-to-amplitude circuit also measures the time between the last clock pulse and the second event. Analog-to-digital converters convert the analog outputs of the time-to-amplitude converters into digital signals. With appropriate scaling, the quantities represented by these digital signals are added to those represented by the counter output, and the result is the desired measurement.
The Chaborski arrangement thus improves the accuracy of the counter-type time-measuring scheme, but it requires the real-time use of analog-to-digital converters.
U.S. Pat. No. 3,970,828 to Klein describes an arrangement that avoids the use of conventional analog-to-digital converters. Like the Charborski circuit, the Klein circuit charges a capacitor between the first event and the time at which the first clock pulse occurs. However, rather than employ a conventional analog-to-digital converter, the Klein arrangement uses the same counter that it uses for coarse measurement as a means for achieving analog-to-digital conversion in the fine measurement.
Specifically, a slow-charging capacitor and a fast-changing capacitor in the Klein circuit both begin charging upon the occurrence of the first event. The fast-charging capacitor is charged only between the first event and the next clock pulse, and it then holds the resultant voltage for comparison with the voltage on the slow-charging capacitor. The slow-charging capacitor keeps charging after the first clock pulse, and the Klein circuit counts the number of pulses that occur in the time that it takes for the slow-charging capacitor to reach the voltage held by the fast-charging capacitor. The slow-charging capacitor charges much more slowly than the fast-charging capacitor does, and many clock pulses occur while the slow-charging capacitor charges if the time between the first event and the next clock pulse is any significant fraction of the clock period. The Klein arrangement therefore determines the duration of the initial time segment very precisely. It measures the terminal time segment similarly, and it thereby makes a precise measurement of the time separation between the events.